Create random keys in Ruby using SecureRandom

Yesterday a friend of mine asked about truly random number generation in Java and which are my thoughts about Random and SecureRandom classes. Of course I told him to use ESAPI calls since they are supposed to be robusts and well designed.

Apart from some weirdness in generation, ESAPI uses Java native SecureRandom facilities, that turns the number generation to be truly random in the computer science meaning.

Let’s see how can we use Ruby SecureRandom class to create truly random keys to use in our software.

Clean SecureRandom API

SecureRandom is a singleton class we can use in our software after requiring it. To accomplish its job, SecureRandom will use:

  • OpenSSL random_bytes routing if OpenSSL has been installed in the machine. It’s not a dependency so, the related gem used to interface with operating system openssl facilities won’t be installed;
  • In Linux or Unix hosts, the /dev/urandom special file is been used. /dev/urandom is a truly random byte container full of entropy directly managed by the operating system kernel.
  • On Windows machines, it uses the CryptGenRandom native method in Windows API

SecureRandom and key generation

It’s very common for a software to need a random key to be used, let me say as salt in String.crypt method or in more sophisticated crypto routines. You may want to have a random key just to use it as temporary password for your users.

We will use SecureRandom to create it one.

require 'securerandom'

def create_key(len=10)

Uh… is it possible just a silly method like that? Yes and not. You may argued, at this point, that our method has a strong limitation: it gives you and hexadecimal charset string, that means you can have [0-9a-f] characters combination.

This is not a true 20 character random string, but looking at some execution it can be safe enough for our purpose.

2.0.0p247 :009 > create_key
 => "bed9741125f193c0308f" 
2.0.0p247 :010 > create_key(20)
 => "19becd8719b6d8e9e2ff9c6c90921a08ce239376" 
2.0.0p247 :011 > create_key(20)
 => "9b177e74fbcf2ae59e7787677b92b93cc42459b3" 
2.0.0p247 :012 > create_key(20)
 => "9a618a7f2686e5e83376ed2c688151cd3d8f24dd" 
2.0.0p247 :013 > create_key(20)
 => "0a299b088442de6eb90956807011a765e24ce4ff" 
2.0.0p247 :014 > create_key(20)
 => "037b967086dbba0a014461106e03e1ed8ff5381c" 
2.0.0p247 :015 > create_key(20)
 => "3adb49097ae0c6d366e53965e45799e41dfc8bae" 
2.0.0p247 :016 > create_key(20)
 => "ab41776af357c42918660f484c11e0c2c2a2570b" 

Off by one

A little cadeaux here.

If you’re on a unix/linux machine, you can go and define a shell function to read data from /dev/urandom for you.

function mkpw() { head /dev/urandom | uuencode -m - | sed -n 2p | cut -c1-${1:-10}; }

$ mkpw 20 
$ mkpw 20
$ mkpw 20
$ mkpw 20
$ mkpw 20

Looking at the execution, this method is far better than the one described before.

Enjoy it!

Image by xkcd

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